Polarization density

Polarization ( or dielectric polarization) is a physical quantity from electrodynamics, which characterizes the strength of the dipole moment in a dielectric material.

Even with non-conductive materials is accomplished by applying an external electric field a shift of electrical charges over short distances in the order of an atomic spacing. In electrical conductors, these shift over much longer distances can be done and is called induction. In both cases, a macroscopic charge distribution ( polarization charges or bound charges ) can be measured on the surfaces.

Mechanisms

Any matter is made of charged components of very different mass. In non-conductors, these blocks are bound to their environment, but can move in different ways anyway:

• With DC voltage dipole molecules can be permanently oriented. Application in the electret.
• At very low frequencies (< 103 Hz) ions may occasionally change places and are retained even after switching off the external field there ( dielectric absorption ). This energy is used, which is why high values ​​assumed. Due to the high mass of the ions these fast field changes can not follow and the effect disappears above 105 Hz
• With increasing frequency dipole molecules are encouraged to periodically flipping at about 1010 Hz - if they exist and are not held in place by a crystal lattice as ice. It is used for example in the microwave between adjacent water molecules to increased attrition.
• Molecules without dipole moment can not be heated in this way and are therefore suitable as an insulating material in high frequency capacitors. In these materials can also be no response at 1010 Hz can be measured.
• At 1012 Hz, the ions oscillate around their equilibrium positions in the molecule. Because while the deflections are limited to a fraction of an atomic diameter, the maximum polarization is quite small. The curved profile is a characteristic sign of resonance and the concomitant phase shift. Resonance is invariably associated with absorption.
• In the area of visible light at 1015 Hz is observed resonances of the electrons in the electric field of the atomic nucleus. This leads to the change of direction of the light waves in glass (refractive index ) and color filters.
• In the UV region at frequencies above 1016 Hz is observed no electrical polarization effects more.

Polarization

Electron polarization: For non-polar molecules, the electron cloud surrounding the nucleus, moved by the applied external electric field against the atomic core. Inside the body is formed as a macroscopic inhomogeneous charge distribution. Once the external field vanishes, the locations of the charge centers are identical again. If it is an alternating electric field (see microwave oven ), is formed by the back and forth oscillation of the core no heat energy.

Orientation polarization

In some types of molecules such as water, the focus of the positive and negative electrical charges are clearly separated. One then speaks of dipole molecules or permanent dipoles whose directions are randomly distributed in the ground state. An industrially important exception are the electrets which contain permanently aligned electric dipoles.

By the action of an external electric field, these dipoles are always better in the same direction, the stronger the field. This polarized mode occurs due to the large masses to be moved slowly, it is also temperature-dependent. A temperature increase in the same direction interferes more and more. With increasing frequency of the electric field, this polarization vanishes first. In contrast, the displacement of the polarization is only weakly dependent on temperature.

Ion polarization

By the electrostatic field, the positive and negative ions of a neutral molecule previously be shifted within the ion lattice together, so that a dipole is formed. Examples are inorganic insulating or ceramic capacitor.

Piezoelectricity

Some dielectrics can generate electrical polarization from mechanical stress. Applications are piezo lighter, force sensors and - because the effect is reversible - quartz oscillators.

Space charge polarization / interfacial polarization

Here, it is assumed that (positive negative ions, electrons ) are provided in a dielectric -free carrier. Without an external field, the individual charges cancel out, and the dielectric acts outwardly electrically neutral. After applying the external field charge carriers move towards the electrode of opposite polarity. It forms a " macroscopic dipole ". Cross- boundary areas may hinder this hike. Charge separation within a layer have the same effect but to the outside. Example: oil-paper insulation, inclusions in the dielectric

Quantitative viewing

Polarization refers to the vector field resulting from a permanent or induced dipole moment in a dielectric material. Here, the polarization vector is defined as the dipole moment per volume.

The dependence of the polarization of the electric field is generally non-linear and anisotropic:

The tensors are - th order is the vacuum permittivity. describes the nonlinear susceptibility is responsible for the Pockels effect and the Kerr effect.

In a homogeneous linear isotropic dielectric medium the polarization is parallel and proportional to the electric field:

Wherein the electrical susceptibility of the medium, i.e., and for.

When the polarization is proportional to the electric field, then the media is not linear mentioned (see also non-linear optics). When the direction of non-parallel to that of, as is the case in many crystals, the medium is anisotropic (see also: Crystal Optics ).

The types of polarization above add up to a total polarization or Gesamtsuszeptibilität:

The individual susceptibilities are frequency dependent. For low frequencies all the parts contribute. At higher frequencies, first the orientation of the polarization disappears ( the molecules with the rapidly changing electric field no longer be free to rotate, such as the microwave range ), then the ionic polarization ( the ions, because of their inertia, the field not follow, such as the infrared range ) and finally, the electronic polarization ( approximately from the UV region ), so that the drops in the high frequency range Gesamtsuszeptibilität to zero.